[Improvement] Unittest Ensuring Hasher Integrity

The data exchanged between Core and GUI are protected by the md5 hash. The md5 functionality was encapsulated into a dedicated function and a unittest to ensure its integrity was added. This is the preparation for update to the new D library md5 API.
[Improvement] Update to New MD5 API

The deprecated std.md5 API replaced with the new one - std.digest.md.
Merge branch 'master' of https://github.com/zippko/SodexoWatchDog

This is no real merge - both of us work on different parts. This is just to
make git happy.